FIGS. 5(a)-5(c) are diagrams showing a structure of a prior art optical semiconductor module comprising an integrated semiconductor laser and modulator, wherein FIG. 5(a), FIG. 5(b), and FIG. 5(c) are a perspective view showing a structure of the integrated semiconductor laser and modulator, a perspective view showing a structure of a substrate on which the integrated semiconductor laser and modulator is placed, and a perspective view showing a structure of the optical semiconductor module comprising the integrated semiconductor laser and modulator placed on the substrate, respectively. Referring now to FIG. 5(a), reference numeral 1001 designates integrated semiconductor laser and modulator (laser element). Reference numerals 1020 and 1040 designate a modulator region and a laser region of the laser element 1001, respectively. Reference numerals 1002a and 1002b designate surface electrodes provided in a surface of the modulator region 1020. Reference numerals 1003a and 1003b designate ground electrodes, respectively, which are electrically connected to a rear surface of the modulator region 1020. Reference numerals 1004 and 1005 designate a surface electrode of the laser region 1040 and a ground electrode electrically connected to a rear surface of the laser region 1040, respectively. Referring to FIG. 5(b), reference numerals 1012, 1006, and 1011 designate a substrate, a microstrip line substrate, and a chip carrier, respectively. The substrate 1012 comprises the chip carrier 1011 and the microstrip line substrate 1006 thereon, the chip carrier 1011 comprising CuW or the like and the microstrip line substrate 1006 being a plate member comprising alumina or the like with a small loss angle to high frequency. The chip carrier 1011 functions as a reinforcement for the microstrip line substrate 1006 and a ground conductor of microstrip lines. Reference numerals 1007a, 1007b, 1007d, and 1007e designate gold-plated strip lines, respectively, disposed on the microstrip line substrate 1006. Reference numerals 1007c and 1007f designate a gold-plated feeding line and a gold-plated ground line, respectively, which are disposed on the micro strip line substrate 1006. Reference numeral 1008 designates a terminal resistor of 50.OMEGA. provided on the strip line 1007b. Reference numerals 1009b, 1009d, 1009e, and 1009f designate through-holes, respectively, whose inner surfaces are metallized by gold-plating or the like, and through which the strip lines are electrically connected to the chip carrier 1011.
Referring to FIG. 5(c), in the optical semiconductor module, the laser element 1001 is placed on the microstrip line substrate 1006 such that their surfaces face each other, and the surface electrodes 1002a and 1002b, the ground electrodes 1003a and 1003b, the surface electrode 1004, and the ground electrode 1005 are bonded to the strip lines 1007a, 1007b, 1007d, and 1007e, the feeding line 1007c, and the ground line 1007f, respectively.
FIG. 6 is a diagram showing a method of constructing a prior art optical semiconductor module. In the Figure, the same reference numerals as those in FIGS. 5(a)-5(c) designate the same or corresponding parts. First, the method of constructing the prior art optical semiconductor module is described with reference to FIG. 6. The method comprises preparing the integrated semiconductor laser and modulator 1001 (laser element)and the substrate 1012; forming AuSn solder bumps 1010 on respective electrodes in the surface of the laser element 1001; placing the laser element 1001 on the microstrip line substrate 1006 that has been heated to be approximately 340.degree. C. such that their surfaces face each other; attaching the surface electrodes 1002a and 1002b and the ground electrodes 1003a and 1003b of the modulator region 1020, and the surface electrode 1004 and the ground electrode 1005 of the laser region 1040, to the microstrip lines 1007a, 1007b, 1007d and 1007e, the feeding line 1007c, and the ground line 1007f, respectively, by means of the AuSn solder bumps 1010, with pressure, and increasing temperature to bond them together, whereby the optical semiconductor module is completed.
Next, a method of fabricating integrated semiconductor laser and modulator (laser element) for use in the prior art optical semiconductor module and solder bumps on the laser element will be described with reference to FIGS. 4(a)-4(f). FIGS. 4(a)-4(f) are cross-sectional views showing a method of fabricating the laser element and the solder bumps in the prior art, which are perpendicular to the laser resonator length direction. In the Figures, reference numerals 1051, 1052, 1062, 1053, 1054, 1055, 1056, 1057, 1058, 1059, 1060, and 1061 designate an n-type InP substrate, a waveguide layer as an active layer in a laser region, a first p-type InP cladding layer, a high-resistance InP current blocking layer, an n-type InP hole trapping layer, a second p-type InP cladding layer, a p-type InGaAs contact layer, a groove by etching, an SiO.sub.2 insulating film, a Cr/Au film, a surface electrode, and a ground electrode electrically connected to the rear surface of the laser element, respectively.
Initially, in the step of FIG. 4(a), on the n-type InP substrate 1051, the waveguide layer 1052 and the first p-type InP cladding layer 1062 are formed by crystal growth, which are etched so as to reach the substrate 1051, to leave a ridge-stripe shaped portion of a prescribed width extending in the laser resonator length direction. Thereafter, the high-resistance InP current blocking layer 1053 and the n-type InP hole trapping layer 1054 are grown to bury the ridge-strip shaped portion, and then the second p-type InP cladding layer 1055 and the p-type InGaAs contact layer 1056 are grown thereon. Note that diffraction gratings generally formed on the waveguide layer 1052 in which the laser region is to be formed are dispensed with herein. In addition, an isolation region to electrically isolate the modulator region from the laser region is also dispensed with herein.
Substantially, in the step of FIG. 4(b), to reduce capacity of the modulator region and, to make electrical connection to the rear surface, etching is performed so as to reach the n-type InP substrate 1051 to form the opening (groove) 1057 in a region comprising no ridge-stripe shaped portion. In the step of FIG. 4(c), after forming the SiO.sub.2 insulating film 1058 over the entire surface of the substrate 1051, immediately on the waveguide layer 1052 and in the deepest portion of the groove 1057, there are formed openings in which the contact layer 1056 and the substrate 1051 are exposed at the bottoms thereof, respectively. In the step of FIG. 4(d), the Cr/Au film 1059 is formed over the entire surface of the substrate 1051. In the step of FIG. 4(e), the Cr/Au film 1059 on the waveguide layer 1052 and the groove 1057, is patterned for separation to form the surface electrode 1060 and the electrode 1061, whereby the laser element 1001 is completed.
Thereafter, in the step of FIG. 4(f), after coating the entire surface of the laser element 1001 with a resist, in a portion thereof a window is made using an exposure technique, and then Au-plating and Sn-plating are sequentially performed thereon and the resist is removed, whereby AuSn solder bumps 1010 with melting point 280.degree. C. comprising alloy with the composition ratio of Au to Sn=4:1 is obtained.
In this optical semiconductor module, the chip carrier 1011 is grounded and potentials of the ground electrodes 1003a, 1003b, and 1005 are set to be zero. In this state, DC current is injected into the laser region 1040 through the feeding line 1007c to generate laser beams therein and a modulation signal is applied to the modulator region 1020 through the microstrip line 1007a to modulate the beams, whereby modulated beams are obtained from the facet.
In the prior art optical semiconductor module, a single substrate comprising alumina with a small loss angle to high frequency is employed as the microstrip line substrate 1006 on which the laser element 1001 is placed and the modulator region 1020 and the laser region 1040 are bonded thereto. However, since heat conductivity of alumina is 0.3 W/cm. .degree.C., and thus low, it is difficult to dissipate heat in the vicinity of emission region generated by injecting current into the laser region 1040, resulting in poor heat dissipation, whereby temperature rises significantly therein. As a result, sufficient light output is not obtained with small amount of current during operation at high temperatures.
On the other hand, in the case of employing a sub-mount comprising a material such as SiC with heat conductivity 2.6 W/cm. .degree.C. as the microstrip line substrate, heat conductivity of the material is thus high but loss angle to high frequency is large, whereby superior high-frequency characteristics which reach several tens GHz from DC is not obtained in the modulator region.
As a result, it is extremely difficult to obtain a high-performance optical semiconductor module which has superior high-frequency characteristics and simultaneously can operate at high temperatures.
FIG. 7 is a perspective view showing a structure of another prior art optical semiconductor module. In the Figure, the same reference numerals as those in FIGS. 5(a)-5(c) designate the same or corresponding parts. An array-type laser element comprising integrated semiconductor laser and modulator (array-type laser element) 2001 comprises a plurality of laser elements 1001 in FIGS. 5(a)-5(c) and they are provided such that their optical axes are disposed in parallel with each other. Note that a ground electrode electrically connected to the rear surface is not provided on the front surface of the laser element 1001 and a rear electrode (not shown) is provided on the rear surface thereof. Reference numerals 2002 and 2003 designate a laser region and a modulator region, respectively. Reference numeral 2005 designates a microstrip line substrate of plate member comprising alumina or the like with a small loss angle to high frequency and on the rear surface thereof a ground conductor (not shown) is provided. Reference numeral 2006 designates plural strip lines through which a high-frequency signal is applied, which are disposed on the microstrip line substrate 2005 such that they extend in the direction perpendicular to the optical axis of the array-type laser element 2001, and whose end portions adjacent to the laser element 2001 are respectively aligned in the optical axis direction. Assume that the number of the strip lines 2006 is equal to that of the laser elements 1001. Reference numeral 2007 designates feeding lines aligned spaced apart equally in the direction perpendicular to the optical axis of the laser element 2001 and they are aligned correspondingly to the laser elements 1001. Reference numeral 2010 designates plural strip lines which are disposed on the microstrip line substrate 2005 such that they extend in the direction perpendicular to the optical axis of the array-type laser element 2001, and whose end portions adjacent to the laser element 2001 are respectively aligned in the optical axis direction of the laser element 2001. The plural strip lines 2006 and the plural strip lines 2010 are disposed on opposite end sides of the laser element 2001. Reference numeral 2008 designates terminal resistors each inserted in each of the plural strip lines 2010 and reference numeral 2009 designates through-holes whose inner surfaces are respectively metallized by gold-plating or the like, through which the plural strip lines 2010 are electrically connected to the ground conductor in the rear surface of the microstrip line substrate 2005.
In this optical semiconductor module, the array-type laser element 2001 is placed on the microstrip line substrate 2005 such that a rear surface thereof is bonded opposite to a surface of the microstrip line substrate 2005. The microstrip lines 2006 are each connected to each surface electrode 1002b of the modulator region 2003, and the microstrip lines 2010 are each connected to each surface electrode 1002a of the modulator region 2003, by means of the bonding wires 2004 such as gold lines or the like. The ground electrodes electrically connected to the rear surface of the laser element 2001 are bonded to ground metal films or the like provided on the microstrip line substrate 2005 where the laser element 2001 is to be placed, and then grounded.
In this optical semiconductor module, the chip carrier 1011 is grounded and a potential of a rear electrode (not shown) of the laser element 2001 is set to be zero. In this state, DC current is injected into the laser region 2002 through the feeding line 2007 to generate laser beams therein and a modulation signal is applied to the modulator region 2003 through the microstrip line 2006 to modulate the laser beams, whereby modulated beams are obtained from the facet.
However, in this optical semiconductor module, with an increase in the number of laser elements 1001 of the array-type laser element 2001, the bonding wires 2004 for connecting the surface electrodes 1002b and 1002a to the microstrip lines 2006 and 2010, respectively, become longer. Accordingly, cross-talk due to transconductance between wirings and self-inductance causes the modulated beams to be distorted. As a result, a high-performance optical semiconductor module with superior high-frequency characteristics is not obtained.
To avoid the problem, it is possible that the strip lines 2006 and 2010 are disposed in the vicinity of the facet from which the laser beams are emitted, i.e., in the vicinity of the modulator region 2003. In this case, however, the bonding wires 2004 prevents the beams from being emitted.
It is also possible that the strip lines 2006 and 2010 are disposed in the vicinity of the facet of the laser region 2002. However, since an output light intensity monitor or a photodiode for controlling an output light is generally provided in the vicinity of the facet of the laser region 2002, it is impossible to dispose the strip lines and provide the boding wires therein.
As described above, in the prior art, a high-performance optical semiconductor module is not obtained.